1. Field of the Invention
The present invention relates to a propylene random copolymer and a film laminate thereof. More specifically, the present invention relates to a propylene random copolymer and a film laminate thereof that has favorable blocking resistance, transparency, and low-temperature heat-sealing properties and that is typically applied as a heat sealant.
2. Description of the Related Art
Polypropylene has many excellent physical properties and is accordingly used in many different applications. Propylene random copolymers, typically used in wrapping films, can be prepared by random copolymerization of propylene with ethylene and/or other .alpha.-olefins and have enhanced heat-sealing properties at low temperatures. These propylene random copolymers are generally produced with conventional Ziegler-Natta catalysts. The propylene random copolymers might have sufficient transparency and heat-sealing properties, which can be attributed to their low crystallinity and low melting point. However, the content of 20.degree. C. xylene soluble fraction (CSX) in the propylene random copolymer, an undesirable property for food wrapping film application, greatly increases with increasing content of ethylene and/or .alpha.-olefin.
In addition, the heat sealant layer that is applied to the surface of the wrapping film is typically treated by corona discharge for improving printing properties. The corona discharge treatment, however, leads to decomposition of the propylene random copolymer in the heat sealant layer, thereby deteriorating the low-temperature heat-sealing properties of the wrapping film.
A method disclosed in JP-A-1-266,116 describes a propylene random copolymer having less content of undesirable 20.degree. C. xylene soluble fraction (CXS) prepared by random copolymerization of propylene with ethylene and/or an .alpha.-olefin in the presence of a catalytic system comprising (1) a group IVB transition-metal compound having one or more cyclopentadienyl groups and (2) an alumoxane. Although the propylene random copolymer prepared by this method has a sufficiently low melting point, the copolymer still has a large CXS content and insufficient blocking resistance. Blocking resistance refers to the extent to which sheets or films stick to each other. Moreover, the catalytic residue of alumoxane is difficult to remove from the resulting propylene random copolymer which undesirably affects the optical properties of the propylene random copolymer.
In addition, a syndiotactic propylene-1-butene random copolymer containing 0.01 to 20% by weight of propylene is described in JP-A-4-175,317. This copolymer has a high content of 1-butene, which results in favorable low-temperature heat-sealing properties but insufficient blocking resistance.
In other work, a syndiotactic propylene-ethylene random copolymer containing 4.6% by weight of ethylene and having favorable low-temperature heat-sealing properties and favorable optical properties is described in JP-A-5-245,992. However, propylene-ethylene random copolymers generally have a large content of 20.degree. C. xylene soluble fraction (CXS) and insufficient blocking resistance, and are thus not suitable for use as a sealant.
To date, propylene random copolymers that possess the contradictory properties of favorable blocking resistance and favorable low-temperature heat-sealing properties have not been prepared.